Ento-phytoscreening: Using insect-induced plant galls to detect anthropogenic volatile organic compounds at urban community gardens

Insect-induced plant galls are sinks for high concentrations of nutrients relative to other plant tissues. By extension, we hypothesize that galls are sinks for contaminants making them valuable bioindicators of contamination such as anthropogenic volatile compounds (aVOCs). These compounds are pervasive contaminants that are often found in formerly industrialized urban areas, such as Detroit, Michigan, USA. These contaminants can persist in groundwater and soil, and negatively affect human health through multiple exposure routes, including agriculture. While plants are often bioindicators for contaminants, in a process referred to as phytoscreening, plant tissues (tree cores, stems, leaves) often fail to concentrate aVOCs at detectable levels using standard laboratory instruments. We propose that ‘ento-phyto-screening,’ using galls may be superior bioindicators to evaluate aVOC exposure risk. Here, we compared the concentration of six aVOCs (BTEX, PCE and TCE) in soil, leaves, stems, cores and galls at seven community gardens in Detroit using two species: grapevine phylloxera, Daktulosphaira vitifoliae (Phylloxceridae: Hemiptera) and hackberry psyllids, Pachypsylla spp. (Aphalaridae: Hemiptera). While the results varied across aVOC compounds, in general we found that galls and leaves had significantly higher concentrations of total aVOCs than soil and tree cores. Galls at only one site accumulated TCE and PCE, suggesting that galls are less likely to detect chlorinated aVOCs with lower volatility. Proximity to known point sources of contamination, and prior site history of the gardens, correlate with increasing concentrations (p=0.035). Our findings suggest that galls could be used as a proxy for human exposure throughout the growing season.